Organic light emitting devices (OLEDs) are large area light sources operated at low voltages of typically 2-5V. OLEDs with large emitting areas require high currents of typically >10 A/m2. To reduce the current requirements one preferred solution is the serial connection of OLEDs on a substrate, where the current passes through the OLEDs connected in series. These serial OLEDs need a smaller current, but a multiple driving voltage. Such large area light sources made of connected OLEDs on a substrate have the disadvantage of gaps between the connected OLEDs where no electroluminescent layer is present. Consequently the gaps are not light emitting and are therefore visible as black lines.
Document US 20050111211A1 discloses a lighting unit with a plurality of separate organic electroluminescent elements each comprising a transparent substrate connected in series. To form a large area light source the side faces (end faces) of the substrates of the electroluminescent element are glued together with an adhesive. Subsequently there is a gap between the electroluminescent elements, where no electroluminescent light is generated. Such a gap would be visible to the outside as a black area. To avoid such black areas US 20050111211 A1 applies a light-scattering adhesive for joining the adjacent electroluminescent elements to emit a part of the electroluminescent light from the adhesive to the outside of the light emitting unit. Therefore the appearance of black areas is at least reduced. The disadvantage of this lighting unit is the required high effort to align and glue together the electroluminescent elements to form a large area light source. Furthermore, the structuring of both electrodes of each electroluminescent unit to enable an electrical series connection of said elements require a multiple masks process applying different masks. The final electrical series connection can only be established after gluing the elements together, which is an additional process step requiring additional effort.